[Adding to John's response]
If you want to understand why, think of what happens if you had this:
1 last first
2 last first
3 last first
And then you deleted the 2nd row to get this:
1 last first
3 last first
Now when you do an insert what would you want the index to be? 3 would certainly be wrong (since it already exists). Do you want it to go find that 2 is missing and thus use 2? What if there are multiple such deleted rows?